Chlorine dioxide-peracetic acid-chlorine dioxide pulp bleaching sequence

ABSTRACT

A CHEMICAL PULP CAN BE BLEACHED WITH CHLORINE DIOXIDE IN THE FIRST STAGE AND PERACETIC ACID IN THE SECOND STAGE TO A BRIGHTNESS OF 60% OF ABOUT. THE ADDITIVE OF A CHLORINE DIOXIDE THIRD STAGE RESULTS IN A FULLY PLEACHED PULPHAVING A BRIGHTNESS OF 80% OR ABOUT IN JUST 3 STAGES, COMPARED TO THE USUAL 5 OR 6 STAGE SEQIEMCES.

sit-108ml US. Cl. 162-67 11 Claims ABSTRACT OF THE DISCLOSURE A chemical pulp can be bleached with chlorine dioxide in the first stage and peracetic acid in the second stage to a brightness of 60% or above. The addition of a chlorine dioxide third stage results in a fully bleached pulp having a brightness of 80% or above in just 3 stages, compared to the usual 5 or 6 stage sequences.

The present invention is directed to a process for the bleaching of wood pulps and, more specifically, to a two or three-stage sequence for bleaching sulfate or sulfite pulps.

The bleaching of chemical wood pulp is normally carried out in multiple stages utilizing elemental chlorine, caustic soda, hypochlorites and chlorine dioxide. The number of stages required in a bleaching sequence depends upon the nature of the unbleached pulp, i.e., sulfite or sulfate, low or high lignin content, etc., and also upon the end use to which it is to be put, i.e., newsprint furnish, dissolving pulp, writing and bond papers. A sulfate or kraft pulp is usually bleached in either 5 or 6 stages which is designated as CEDED or CEHDED. C denotes elemental chlorine, E denotes caustic extraction, D denotes chlorine dioxide and H denotes hypochlorite bleaching. Conventional kraft bleaching utilizes a large volume of water, particularly in the chlorination stage, since chlorination is usually done at 3% pulp consistency. Bleaching in this conventional fashion generates objectionable chloride ions and consequently the disposal of process efiluent causes a severe stream pollution problem. Capital cost for a bleach plant utilizing the conventional bleaching sequences, as described above, is quite high, because of the large number of pieces of equipment which must be used. Exemplary thereof are washers, retention towers, centrifugal pumps, dilution chests and control instruments. By following either of the multistage bleaching processes described above, considerable fiber damage results, whereby there is a bleached yield loss of at least 6-7%.

It is an object of the present invention to provide a process wherein wood pulp is bleached in a 2-stage sequence to a brightness of 60% or above.

It is another object of the present invention to provide a process wherein wood pulp is bleached in a 3-stage sequence to a brightness of 80% or above.

It is still another object of the present invention to provide a pulp bleaching process which reduces the amount of chloride ions in the process efiluent.

It is still yet another object of the present invention to provide a pulp bleaching process which markedly reduces stream pollution.

It is a further object of the present invention to provide a pulp bleaching process wherein the bleached pulp has a high viscosity and is produced in high yield.

It has been found that when a chemical pulp is bleached in a first stage with chlorine dioxide, either gaseous or in solution, followed by a second stage emp'oying peracetic acid, a pulp having a brightness of 60% or above is obtained. If the peracetic acid second stage bleach is followed by a third stage employing chlorine dioxide, either gaseous or in solution, the resulting pu'p is fully bleached in just 3 stages and has a brightness in excess of 80%.

The use of the three-stage bleaching sequence, namely, chlorine dioxide-peracetic acid-chlorine dioxide (D-Pa-D), results in a number of advantages. One, the brightness achieved in just 3 stages is comparable to that achieved in a conventional 5-stage sequence. This should reduce the capital costs of new bleach plants considerably. Two, the pulps are of better quality, i.e., higher viscosity, and higher bleached yield, i.e., less pulp shrinkage, when compared to pulp bleached by a fivestage process. Three, the present process will be advantageous with respect to heat economy and water requirements, since all stages can be carried out at the same temperature and pulp consistency. Fourth, since the consistency of the pulp when bleaching with chlozine dioxide can be from 10% to 40% as opposed to the 3% pulp consistency which is required when bleaching with ele: mental chlorine, water'requirements for the system will be greatly reduced and, consequently, the stream pollu tion load will be decreased. 0 I

In the first stage, an unrefined, digested sulfate or. sulfite pulp is treated with either a solution of chlorine dioxide in water or with gaseous chlorine dioxide.

When a solution of chlorine dioxide in water is used,

the consistency of the pulp can be from about 10% to about 20%, based on the weight of the oven-dried pulp, and preferably between. 10% and The concentration of the chlorine dioxide can be from about 0.5% to about 3.0%, based on the weight of the oven-dried pulp. It is preferable that the concentration of chlorine dioxide be 1.5% for hardwood kraft pulps, 2% for softwood kraft pulps and 1% or below for sulfite pulps, all percentages being based on the weight of the oven-dried pulp. The reaction can be carried out at a temperature between about C. and about 90 C., with a temperature of 70 C. being preferred, and at an initial pH between about 5.0 and 6.0. When, for example, 1.5% chlorine dioxide and a, temperature of 70 C. are employed, a typical reaction time is two hours.

When gasoeus chlorine dioxide is used in the first stage, the pulp consistency can be from about 10% to about with a pulp consistency above 30% being preferred. When gaseous chlorine dioxide is employed, having a concentration of from about 0.5 to about 3.0%, based on the weight of the oven-dried pulp, the reaction time is from about 5 to 10 minutes and the reaction takes place at approximately 20 C.

By the use of a solution of chlorine dioxide or gaseous chlorine dioxide in the first stage, under the process conditions set forth above, the lignin level of the pulp is markedly reduced, as evidenced by a reduction in the Kappa No. from about 15 down to about 2 to 4.

After water washing followed by centrifugation or filtration, the partially bleached pulp of the first stage is subjected in the second-stage to bleaching with peracetic acid. The concentration of the peracetic acid can be from about 0.5% to about 2.0%, based on the weight of the oven-dried pulp, and the pulp consistency can be from about 10% to about 20%, and preferably between 10% and 15%. The rcactioncan be carried out at a temperature of from about C. to about C., with a temperature of about 70 C. being preferred. During the peracetic acid second stage, which can take from about 30 minutes to about minutes, the inital pH must be within the range of pH 8.0 to 12.0 and preferably between pH 8.0 and 10.0. The pH can be adjusted by adding NaOH and/or Na CO The initial pH in the second stage must be adjusted to 8.0 or above to obtain optimized bleaching results.

After the first two stages, namely, chlorine dioxide (D) followed by peracetic acid (Pa) the pulp has a brightness of 60% or above and is suitable for use as newsprint furnish.

In order to increase the brightness of the pulp to 80% or above, so that it may be utilized in the manufacture of dissolving pulp or bond papers, the pulp is treated in a third stage with chlorine dioxide, either in solution or in the gaseous state.

The concentration of the chlorine dioxide in solution can be from about 0.25% to about 2.0%, preferably 1%, and the pulp consistency can be from'about to about preferably between 10% and 15%, all percentages beingbased on the weight of the oven-dried pulp. The reaction temperature can be from about 50 C. to about 90 C., with 70 C. being preferred and the reaction time can be from minutes to 120 minutes.

In certain instances it would be'advantageous to introduce a caustic soda extraction stage between the chlorine dioxide first stage and the peracetic acid second stage. In pulps having high lignin contents, e.g., 5% or more (softwood kraft pulps), the addition of from about 1% to about 2% of NaOH, based on the oven-dried weight of the pulp, at about 70 C. for about 2 hours, results in increased pulp brightness in the subsequent stages.

Thefollowing examples illustrate the process of the present invention.

EXAMPLE 1 Onehundred grams, oven-dried basis (O.D.), of hardwood pulp, prepared by the kraft process, was treated in the first stage as follows.

1st stage:

Chlorine dioxide (solution) 1.5% on 0.13. pulp. Reaction temperature 70 C. Reaction time 2 hours. Starting pH 5.5-6.0. Pulp consistency 10.0%.

The pulp sample was then thoroughly washed with water and was then centrifuged.

The partially bleached sample was then evaluated. The results are shown in Table 1 below.

TABLE 1 Partially bleached Unbleached Kappa No. (TAPPI T236, m-) Viscosity (TAPPI T230, su-GG) (eentipoise Brightness (TAPPI T217, m-48) (percent) It can be seen from Table 1 that by treating the unbleached kraft pulp with chlorine dioxide (D) in the first stage, there results a reduction in Kappa No. and a brightness increment of 20%.

The above partially bleached pulp (D), i.e., first stage, was bleached with peracetic acid in the second stage as 60 follows.

The above chlorine dioxide-peracetic acid (D-Pa) pulp was then washed with water and centrifuged. The semibleached P p was evaluat d The results a shown i T b e 2 b low. 7

TABLE 2 Semi- Unbleached bleached (D-P a) Kappa No 14. 75 2. 62 Viscosity. 26. 74 22. 47 Brig htness 32. 4 79. 7

The results shown in Table 2 clearly demonstrate that a kraft pulp can be bleached in only two stages, using chlorine dioxide in the first stage and peracetic acid in the second stage to a brightness suitable for newsprint furnish and also for the semi-bleached market.

The semi-bleached pulp (D-Pa) of the second stage was then bleached with chlorine dioxide (D) in third stage as follows.

3rd Stage:

Chlorine dioxide (solution) 0.5% on 0D. pulp. Reaction temperature 70 C. Reaction time 2 hours. Starting pH 6.0-6.5. Pulp consistency 10.0%.

The fully bleached pulp of third stage was then washed with water and centrifuged. The evaluation of the fully bleached pulp is presented in Table 3 below.

TA B L E 3 Fully Unbleached bleached (D-Pa-D) Viscosity 26. 74 24. 99 Brightness. 2.4 88.4

TABLE 4 Bright Viscosity ness (centi- Shrinkage Fully bleached (percent) poisos) (percent) ---stage D-Pa-D (Example 1) 88. 4 25. 1 3. 5-stage CDEDED (conventional) 85. 5 17.0 5. 53

It can be seen from Table 4 that the D-Pa-D sequence results in a pulp having higher viscosity, i.e., better physical properties, and less pulp shrinkage, i.e., less bleached yield loss, when compared with a pulp bleached by the conventional C EDED sequence.

EXAMPLE 2 One hundred grams, oven-dried basis (O.D.), of hardwood pulp, prepared by the kraft process, was treated in the first stage with gaseous chlorine dioxide, rather than with chlorine dioxide in solution as in Example 1. The conditions in each of the three stages are listed below.

1st Stage:

Chlorine dioxide (gaseous) 1.31% on O.D. pulp. Reaction temperature 20 C. Reaction time 5 minutes. 2nd Stage:

Peracetic acid 1% on CD. pulp. Reaction temperature 70 C. Reaction time 2 hours. Starting pH 8.0.

The brightness of the pulp after the first two stages, namely, gaseous chlorine dioxide and peracetic acid (Dg-Pa) was measured. See Tab 5 for resu ts.

5 3rd Stage:

Chlorine dioxide (solution) 0.7% on O.D. pulp. Reaction temperature 70 C. Reaction time 2 hours.

The pulp was washed with water and then centrifuged between each of the stages. The pulp was then evaluated. See Table 5, No. 3.

What is claimed is:

1. A two-stage process for bleaching chemical pulps, without previous chlorine or hypochlorite treatment, to a brightness of 60% or above, which comprises:

(a) a first stage of reacting said pulp with from about 0.5% to about 3.0% of chlorine dioxide, based on the weight of the oven-dried pulp, at a temperature of from about 20 C. to about 90 C. for a period of from about 5 minutes to about 120 minutes;

(b) a second stage of reacting the pulp with from about 0.5% to about 2.0% of peracetic acid, based on the weight of the oven-dried pulp, at a temperature of from about 50 C. to about 90 C. for a period of from about 30 minutes to about 120 minutes.

2. The process as recited in claim 1 wherein the starting pH of the peracetic acid sec-nd stage is from about 8 to about 12.

3. The process as recited in claim 1 wherein the starting pH of the peracetic acid second stage is from about 8 to about 10.

4. The process as recited in claim 1 wherein the chlorine dioxide in the first stage is gaseous chlorine dioxide, with a pulp consistency, based on the weight of the ovendried pulp, of from about 10% to about 40% and the reaction temperature is about 70 C. for a period of from about to about minutes.

5. The process as recited in claim 1 wherein the chlorine dioxide in the first stage is chlorine dioxide in solution, with a pulp consistency, based on the weight of the oven-dried pulp, of from about 10% to about and the reaction temperature is about 70 C. for a period of from about to about 120 minutes.

6. A three-stage process for bleaching chemical pulp, without previous chlorine or hypochlorite treatment, to a brightness of 80% or above, which comprises:

(a) a first stage of reacting said pulp with from about 0.5% to about 3.0% of chlorine dioxide, based on the weight of the oven-dried pulp, at a temperature of from about 50 C. to about 90 C. for a period of from about 5 minutes to about 120 minutes;

(b) a second stage of reacting the pulp with from about 0.5% to about 2.0% of peracetic acid, based on the weight of the oven-dried pulp, at a temperature of from about C. to about C. for a period of from about 30 minutes to about minutes;

(c) a third stage of reacting the pulp with from about 0.25% to about 2.0% of chlorinedioxide, based on the weight of the oven-dried pulp, at a temperature of from about 50 C. to about 90 C. for a period from about 30 minutes to about 120 minutes.

7. The process as recited in claim 6 wherein the starting pH of the peracetic acid second stage is from about 8 to about 12.

8. The process as recited in claim 6 wherein the starting pH of the peracetic acid second stage is from about 8 to about 10.

9. The process as recited in claim 6 wherein the chlorine dioxide in the first and third stages is gaseous chlorine dioxide, with a pulp consistency of from about 10% to about 40%, based on the weight of the oven-dried pulp, and the reaction temperature is about 70 C. for a period of from about 5 to about 10 minutes.

10. The process as recited in claim 6 wherein the chlorine dioxide in the first and third stages is chlorine dioxide in solution, with a pulp consistency of from about 10% to about 20%, based on the weight of the oven-dried pulp, and the temperature is about 70 C. for a period of from about 30 minutes to about 120 minutes.

11. The process as recited in claim 6 and including a caustic soda extraction stage between said chlorine dioxide first stage and said peracetic acid second stage, said caustic soda having a concentration of from about 1% to about 2%, based on the weight of the oven-dried pulp.

References Cited UNITED STATES PATENTS Yorston et al. l6278 X S. LEON BASHO RE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. 

